A typical wireless communication system comprises a plurality of wireless communications devices exchanging data with each other. In some wireless communication systems, for example, infrastructure networks, the system may further comprise a wireless base station for managing communications between the wireless communications devices. In other words, each intra-system communication would be exchanged via the wireless base station. In other wireless communication systems, for example, mesh networks and ad hoc wireless networks, the wireless base station may be omitted, i.e. the wireless communications devices may communicate directly with each other.
A typical Extremely High Frequency (EHF), i.e. 30 to 300 GHz, communication system may have some drawbacks. For example, transmission of the signals over coaxial cable may incur large attenuation effects. Moreover, in applications where RF devices are used, the size, weight, and power (SWaP) of the components may increase to undesirable levels. Moreover, downstream receiver processing, such as downconverting, and signal addressing may be difficult.
One approach to these drawbacks in EHF communication systems may comprise the use of optical components for signal processing. An advantage of such systems may comprise the ability to transmit EHF signals from a remote location without the degradation of the signal incumbent in RF applications.
For example, as disclosed in U.S. Pat. No. 5,710,651 to Logan, Jr., an EHF communication system comprises a remote antenna station, a transmitter/receiver station, and an optical fiber coupling the stations together. These stations comprise photodiodes for converting the transmitted optical signal to an electrical signal (receiver station), and lasers paired with optical modulators for converting the received EHF signal to an optical signal (transmitter station).
In optical communication system applications, it may be desirable to maintain a high degree of linearity at EHF ranges, in particular, 30-60 GHz and large instantaneous bandwidth, such as 0.1-4 GHz. These applications typically include the use of an external modulated RF photonic transmit link. One potential limit to performance in these applications is the linearity of the optical intensity modulator.
One approach to this drawback is disclosed by Marpaung et al., “A photonic chip based frequency discriminator for a high performance microwave photonic link,” Optics Express, Vol. 18, No. 26. This device includes a single continuous wave (CW) laser, a frequency or phase modulator coupled to the CW laser, a pair of complimentary frequency modulation (FM) discriminators, and a waveguide coupling together the FM discriminators and the modulator. This device provides a wideband signal with linearity improvements. Nevertheless, this device has increased complexity and power consumption since the FM discriminators are thermally controlled.
Another approach is disclosed in U.S. Pat. No. 6,246,500 to Ackerman. This optical link device includes a pair of optical sources, a multiplexer coupled to the optical sources, an intensity modulator (Mach-Zhender modulator) coupled to the multiplexer, and a receiver end coupled to the modulator via a waveguide. This optical device provides 8 dB in dynamic range improvement at 1 GHz, but uses different electro-optic coefficients of the Mach-Zhender modulator and precise photocurrent control to reduce distortion. One potential drawback to this approach is that the needed precise control of the optical carrier intensity and/or polarization may be problematic. Another approach similar to that of Ackerman is disclosed in U.S. Pat. No. 7,079,780 to Rollins. This optical link device uses a low biasing technique, i.e. a narrowband biasing technique.
Yet another approach is disclosed by Darcie et al., “Class-B microwave-photonic link using optical frequency modulation and linear frequency discriminators,” Journal of Lightwave Technology, Vol. 25, No. 1. This optical link device includes a pair of complimentary fiber-based linear filters coupled to respective modulators. Nevertheless, this optical link device may provide only narrowband performance and a 3 dB improvement in third order intercept point (OIP3).